It has been long known in the prior art to employ aromatic diamines, including various unsubstituted or substituted methylene-dianilines as curing agents for polyurethanes as well as for epoxy resins. Ring substituents in the aniline molecule have included halogen, as well as alkyl, alkoxy, and carboxyalkyl groups. The resulting diamine type curing agents have been found essential to impart strength and toughness to the resins to which they are applied. However, the known diamine type materials differ widely in their curative action, and most are too fast-acting to be of practical importance.
At the present time, the most widely used curing agent for the foregoing types of polymers is the compound 4,4'-methylene bis-orthochloroaniline, which is available commercially under the designations MOCA and Curene 442. Owing to steric hindrance and electronic effect of the amino group, this diamine possesses satisfactory pot life and also imparts excellent physical properties to the resin products with which it is employed. However, this compound is toxic, and is also a suspected carcinogen. Recently promulgated federal regulations relating to occupational safety and health restrict work-place exposure to this substance.
Another diamine which has been employed for curative purposes in connection with diamine cured polyurethane compositions is methylene bis methyl anthranilate, known as MMA, the use of which is mentioned in U.S. Pat. Nos. 3,188,302 and 3,629,168. However, this substance exhibits a very short pot life and imparts poor physical properties to urethane elastomers.
Another disadvantage of the agent 4,4-methylene bisorthochloroaniline is its tendency to undergo discolorization when kept in the molten state for prolonged periods, such as several hours. This may adversely affect the color of the polyurethane vulcanizate, and may require the addition of a color stabilizer such as an alkali metal hydroxide thereto, as described in U.S. Pat. No. 3,393,239.
The conventional method of preparation of the methylene dianiline curing agents of the prior art has been to condense formaldehyde with one or more substituted anilines under acid conditions, and in specified molar ratios. Where more than one aniline or substituted aniline is reacted, it is not certain whether a single molecular compound is produced, or rather a mixture of individual diamines each derived from reaction between the formaldehyde and a particular aniline. In some instances the products are believed to be mixtures of isomeric polyamines. Hence such curing agents are conventionally characterized as prepared by condensing the various compounds in the presence of an acid in specified molar ratios.
The diamine curing agents are conventionally blended with the polyurethane prepolymers by uniformly dispersing them therein, either directly or via a liquid diluent. They are similarly blended with uncured epoxy resins.